When fabricating large structures such as external propellant tanks for the Space Shuttle, large sheets of aluminum alloy are curved and welded along joints to define the overall shape of the tanks. The welding is preferably performed by automatic welding machinery, such as, for example, that described in U.S. Pat. No. 5,483,039, issued Jan. 9, 1996 in the name of Gallagher. It has been found that stress relieving or planishing of the welded joint is advantageous, as described, for example, in U.S. patent application Ser. No. 08/803,481, filed Feb. 20, 1997 in the name of Shah et al. As described therein, planishing is accomplished by use of a planishing hammer applied to the weld on one side of the structure, and a backing or bucking bar held in a corresponding location on the other side of the structure.
It will be appreciated that the metal from which the propellant tank of the Space Shuttle is fabricated may, in some locations, be relatively thin, for weight minimization. During the planishing operation, the backing bar must be applied to the reverse side of the plates being welded, to prevent damage or actual puncturing of the plates by the hammer. As mentioned, the tanks are very large, so a human operator and either a backing bar or a power hammer can easily be accommodated within the tank, and the curvature of the plates is so large that, at any particular location, the joint or weld to be planished lies in an essentially planar structure.
It has been found to be difficult to reliably maintain the backing bar at a position on one side of the structure which corresponds to the location of the head of the planishing hammer. While the weld is visible from both sides of the structure, the exact position along the weld which is being planished must be identified to within the diameter of the backing bar. Attempts have been made to use radio communications to give instructions and information across the welded walls, but this has not proven to be effective.
Improved arrangements for identifying the location of a planishing hammer are desired.